Dough – What type of flour is ideal for sourdough

You can easily replace the liquid in most bread recipes with beer. This can have a very pronounced effect on your final dough as there is a lot more chemical and biological fun happening in beer than there is in water. In my experience, the dough with beer will usually rise faster than a similar dough with water. Generally, the flavor difference won't be that pronounced (usually a much more "bready" flavor, unless you use a beer that otherwise has a strong flavor, such as an IPA).

The acidity of the beer won't actually have that much of an effect on your final dough as the ph will be a weighted average of all of your ingredients.

A lot of sourdough recipes don't call for a sponge, per se, because the starter essentially already is a sponge. The purpose of a sponge in a bread with normal yeast is to allow a longer fermentation time and extra time for enzymatic reactions to happen in the flour before adding it to the final batch. This isn't necessary with sourdough the starter is continuously fermenting and reacting.

And your guess is about right, if the proportion of starter you add is too high, the waste products and dead yeast cells in it can cause changes in the gluten that make it very slack and unusable.